FeX has been found to replicate in 13 of 13 feline embryonic cell lines tested. It is also a common contaminant of type specific FeLV stocks and other viruses which have been grown in cat cells. At least two intracellular control mechanisms for FeX expression in cat cells have been detected. A spontaneous transformed cat cell line containing LETTS protein has been isolated. It appears to have lost another major surface protein present in its normal counterpart.